1. Field of the Invention
The present invention relates to an apparatus and method for measurement of a cardiovascular function. In particular, the present invention relates to a totally noninvasive, quick and simple medical instrument for measurement of cardiovascular characteristics during voluntary cardiorespiratory synchronization.
2. Description of the Prior Art
Despite the extensive research relating to the cardiovascular system and despite the development of sophisticated electromedical instrumentation, there is a continuing need for a simple, reliable, noninvasive and rapid method of providing an indication of the condition of the cardiovascular system. At the present time, the most commonly used type of equipment is the electrocardiogram (EKG), in which several electrodes are attached to the subject and the signals are amplified and used to generate an electrocardiogram. The interpretation of an electrocardiogram requires a skilled physician, who must make a number of qualitative as well as quantitative decisions to determine whether or not the subject's cardiovascular system is within a normal range. The electrical signals comprising the EKG record convey information which is strictly limited to the electrical activity of the cardiovascular tissue. While changes in the depolarization and repolarization patterns of cardiac tissue are associated with certain pathological conditions, no direct physiologic information is available from the EKG record. A simpler, easier-to-use system which provides a more direct information about the physiologic cardiovascular condition would be highly desirable.
Two and one-half centuries ago, Stephen Hales documented respiratory influences upon the circulatory system. Hales, S., Statical Essays, Vol. II Haemastaticks. (Innins and Manby, London, 1733). In 1847, Karl Ludwig recorded concurrent heart rate, respiratory pattern, and blood pressure in a dog and noted that during inspiration the heart rate increased and blood pressure fell while the opposite occurred during expiration. Ludwig, F. W. "Beitrage zur Kenntniss des Einflusses der Respirations-bewequngen auf den Blutlauf im Aortensysteme". Achiv fur Anatomie und Phsyiologie, 261, 1847. Over the years, the misnomer "sinus arrhythmia" (S.A.) has been attached to this ryhthmic phenomenon of pulse quickening upon inspiration and slowing upon expiration. Hering, E., "Uber eine reflectorische Beziehung zwischen Lunge und Herz". Sitzber. Akad. Wiss. Wien. 64:333-353, 1871.
The precise causes of this relationship have been investigated since these early observations. The proposed mechanism include: rhythmically changing sympathetic and parasympathetic nervous balance; respiratory phase-dependent changes in blood chemistry; and recurrent breathing-dependent changes in the intrathoracic pressure. These rhythmic pressure changes affect cardiac filling. Akselrod, S., et al., "Power Spectrum Analysis of Heart Rate Fluctuation: a Quantitative Probe of Beat-to-Beat Cardiovascular Control", Science 213:220-222, 1981. Williams, A., Copper, A., "Interrelationships of Cardiac Output, Blood Pressure, and Peripheral Resistance During Normal Respiration in Normotensive and Hypertensive Indiviudals", Circulation 4:278-286, 1951. Chess, G., Tam, R., Calaresco, F, "Influence of Cardiac Neural Inputs on Rhythmic Variations of Heart Period in the Cat", Am. J. of Physiology 288:775-781, 1975. Womak, B., "The Analysis of Respiratory Sinus Arrhythmia Using Special Analysis and Digital Filtering", IEEE Transactions on Biomedical Engineering 18:339-409, 1971. Flaherty, J., "Influence of Respiration on Recording Cardiac Potentials", Am. J. of Cardiology 20:21-28, 1967. Fischmann, E., et al., "Beat to Beat and Observer Varation of the Electrocardiogram", American Heart Journal 75:465-473, 1968. Lauson. H., Bloomfield, R., Cournand, A., "The Influence of the Respiration on the Circulation in Man", Am. J. of Medicine 1:315-336, 1946. Starr, I., Friedland, C., "On the Cause of the Respiratory Variation of the Ballistocardiogram, with a Note on Sinus Arrhythmia", J. of Clin. Investiation 25:53-64, 1946. Melcher, A., "Respiratory Sinus Arrhythmia in Man", Acta Physiologica Scandinavica 435:6-25, 1976. In 1920, Bainbridge proposed that the greater the negative intrathoracic pressure generated by each inspiration, the greater the cardiac filling and quicker the subsequent pulse. Bainbridge, F. A., "The Relationship Between Respiration and the Pulse Rate", J. Physiol. 54:192-202, 1920.
Medical students have been taught for many decades that the sinus arrhythmia disappears with advancing age. Were this "disappearance" gradual rather than abrupt and quantifiable, it might provide a measure of cardiovascular physiologic age. Any index sensitive enough to accurately quantitate physiologic aging might also have uses in defining biologic effects of cardioactive drugs and cardiac toxins. Such an index must be equally useful in serially evaluating improvement or deterioration of cardiac tissue. Precise quantification of the sinus arrhythmia, however, has been hampered because the resiratory cycle and heart rate bear no fixed relationship to one another in the uncontrolled situation.
In 1963, a system called "Voluntary Cardiorespiratory Synchronization" (VCRS) was devised whereby breathing and heart rate could be voluntarily synchronized. This system was subsequently described in Almasi, J., Schmitt, O., "Basic Technology of Voluntary Cardiorespiratory Synchronization in Electrocardiology", IEEE Transactions on Biomedical Engineering 21:264-273, 1974. This system allows the subject to lock respiratory and heart rates in phase at a fixed integral ratio.
The principal features of the VCRS method used by Almasi and Schmitt included EKG leads which formed a pulse sensor, a current discriminator which counted successive electrical impulses which corresponded to sensed heart beats, and a ring counter which could be set to control light driver circuits at any desired ratio of sensed heart beats. The light driver circuits converted the desired pattern to a visual signal which advised the subject to inhale or exhale, each for the pre-selected number of beats. The system was "closed" when the subject voluntarily breathed according to these visual instructions. Thus respiration and heart beats were synchronized. If the subject could follow these simple visual instructions and the heart was in sinus rhythm, any reasonably comfortable pulse: inhale; pulse: exhale ratio could be achieved. The results of this VCRS method were recorded in the form of an EKG record. The VCRS circuit developed by Almasi and Schmitt has found no clinical application because the manual collection of instantaneous heart rate data from the cardiograph paper provides a record which is so cumbersome as to be completely useless in the clincal sense.